With the term “exhaust air” we mean the air flowing from the drying chamber, i.e. in the proximity of the air outlet from such chamber. With the term “drying temperature” we mean the reference temperature for controlling the drying process, including the control of the heating element used for heating air entering the drying chamber.
A common practice is to control a tumble dryer heating element by feeding back the exhaust air temperature. The drum output temperature is usually a good approximation of the actual clothes temperature, therefore it is kept under control to avoid an excessive heating of clothes which could damage them.
The feedback is usually made through hysteresis control, i.e. the heater is switched on when the feedback temperature is below a first predefined threshold and switched on when it is above a second predefined threshold. In this way the hysteresis control shows low performance when the temperature of the heater is around the upper temperature limit and it may cause undesired oscillation of the clothes temperature.
Another more advanced way to control the heater is through a PI (proportional-integral) control and PWM (Pulse Width Modulation) control.
In the attached FIG. 1 classic control of a domestic tumble dryer is shown where the input of the control algorithm is the difference between the drum output temperature set point and its current value. The algorithm may be a simple hysteresis control or a PI control, where the output directly manages the heater actuation.
The exhaust temperature set point is fixed and for this reason the control performances are strongly dependent on the working operation conditions. Hence the time/energy performances depend on the mass of the clothes inside the dryer, the water retained by the load, the venting condition and the environment condition.